All over the world, there are parcels of land where the soil and groundwater have been impacted by hydrocarbons such as diesel fuel and gasoline. These contaminants have the potential to migrate into surface water and groundwater, affecting the environment as well as human water supplies. The prospect of high remediation costs means that many otherwise-promising “brownfield” properties in previous industrial locations remain undeveloped.
Because of the importance of this issue, Golder is carrying out research on ways to remediate soil impacted by Light Non-Aqueous Phase Liquid (LNAPL) petroleum hydrocarbon constituents, which include many common petroleum products. Golder’s research focuses on how natural remediation processes, which we might collectively call “Mother Nature,” can dovetail with active, human-engineered technologies.
Understanding what Mother Nature brings to the process has the potential to reduce costs and shorten remediation timelines, allowing the beneficial use of properties that are now just growing weeds.
How Natural Source Zone Depletion (NSZD) fits into the picture for impacted soil betterment
The remediation techniques now used for LNAPL-impacted soil include dig-and-dump – digging up the problematic soil, taking the problem soil offsite in trucks and depositing it in landfills designed to keep harmful materials locked away. This method is becoming less tenable because of rising costs, carbon impacts of the truck traffic, and increasing reluctance of landfill operators to accept impacted soil.
Alternatives to dig-and-dump include a growing range of “in-situ” techniques such as enhanced bioremediation, where natural processes are augmented with agents to boost microbial activity. Other options include injection of oxidants or physical stripping processes such as soil vapour extraction or air sparging.
One disadvantage is that these techniques may have impacts of their own, such as the fact that some technologies produce wastes that must be treated, and carbon emissions produced by remediation equipment and vehicles. As well, the effectiveness of most treatment techniques tends to decline over time.
Accordingly, Golder has been researching what Mother Nature offers through Natural Source Zone Depletion (NSZD), a term used to describe the naturally occurring processes of dissolution, volatilization and biodegradation that result in mass losses of LNAPL constituents from the subsurface. NSZD is seeing growing interest partly because of research carried out by Golder and other organizations, which shows that Mother Nature has the powerful ability to deal with LNAPL-impacted soils.
The emerging research is finding that NSZD can remove LNAPL at rates similar to, or in some cases even greater than, the rates achieved during the later stages of remediation by engineered (active) remediation systems.
Golder’s research highlights four major roles for NSZD:
- As a baseline to evaluate natural conditions and the effectiveness of and need for engineered remediation systems, including enhanced bioremediation, where a boost to NSZD may be enough to meet regulatory standards within the specified timeline;
- As a component of a remediation plan, where NSZD is relied upon for less-impacted areas of the site, and engineered remediation systems can be focused on the more problematic parts of the property in question;
- As a stand-alone remedy, where impacts caused by LNAPL are stable, receptors (such as potable groundwater, a wetland or watercourse) are not at risk, and timeframes can be shown to likely meet the business goals of the site owner as well as regulatory requirements; and
- As a final step in a remediation program for a site, where NSZD can finish the job after other remedial technologies have achieved their design objectives.
Why it’s important to learn to work with Mother Nature through NSZD
The emerging understanding is that depletion of the mass of LNAPL-impacted soils through NSZD can be significant. This means that incorporating NSZD into the treatment plan can be a viable and cost-effective risk-management approach at many sites.
Therefore, comparison of active and natural depletion rates at mature and stable source zones can be an effective strategy for evaluating remedial alternatives, and for establishing criteria for technology transition from active to natural remediation as an overall more sustainable management strategy.
This calls for better ways to calculate the boost that Mother Nature can provide in treating impacted soil and groundwater. This includes understanding the difference between natural processes for source zone depletion above the water table, where aerobic (involving oxygen) processes occur, and those below the water table, where the water in the soil means a preponderance of anaerobic processes.
Measuring the rate of these processes – indicating the speed of remediation – is helped by the fact that both aerobic and anaerobic biodegradation produce carbon dioxide (CO2). This accumulates in soil gas (gasses trapped among soil particles) and migrates out of the soil as efflux at surface. Golder is studying how best to measure the amount of CO2 efflux as a way to understand the rate of NSZD occurring at a specific location.
The respiration of natural soil organics also produces CO2, and this can cause problems for interpretation of CO2 concentrations found in efflux from the soil. Golder is working on ways to apply radiocarbon dating to this problem. This relies on the fact that carbon-14 (C14), a carbon isotope, tends to become depleted in carbon over time, so it is not present in carbon found in fossil fuels such as gasoline. “Modern” carbon is comparatively rich in C14. This means that determining the C14 content or fraction of radiocarbon is a way to see how much of the carbon comes from LNAPL, helping to calculate how much of the contaminant is being depleted.
Golder’s work on applying these technologies to real-world situations is helping to push forward an understanding of factors influencing the abilities of Mother Nature to contribute to the cleanup of impacted soil.
This may also have the benefit of helping property owners and governments build a better business case for remediating properties that would otherwise stay undeveloped.
For more on Golder’s research focus on natural remediation processes, read about our research into plant-remediation and a successful case study on using trees to effectively remediate contaminated soil and groundwater.